Diseases originated in the gastrointestinal tract (GI tract), especially malignant tumors such as the colon cancers, pose daunting threats to human health. Computed Tomography (CT) is widely used for the medical imaging diagnosis of GI tract diseases. In recent years, with the application of the multi-planar spinal CT instrument with scanning speed at sub-second range, the GI imaging qualities have greatly improved with much fewer artifacts resulted from breathing and intestine peristalsis. Therefore CT plays a more and more important role in the diagnosis of GI tract, especially colon, diseases. In current clinical practice, air and water are usually used for the filling of the intestine to fully display of the anatomic structures, in order to improve the detection accuracy and reduce false positives. In addition, CT virtual endoscopy techniques for three dimension (3D) image reconstruction and analysis were developed to further improve the diagnostic specificity and sensitivity for GI tract diseases. But the early and accurate detection of colon tumors that originated at the colon wall was still not possible by CT imaging only. The most important reason for such a limitation is that imaging contrast agents currently available are either positive contrast agents (with high CT densities) or neutral contrast agents (with CT densities similar to that of water). However, the positive contrast agents tended to conceal the small changes on the intestinal wall, and the high CT density in the lumen may interfere with the colon wall images. The neutral contrast agents could not show clearly the anatomical structures of the intestinal wall, intestinal lumen and related tissues, because their CT densities are not much different from the wall, so the contrast is low even with full filling of the colon. There is usually a high missing rate in diagnosis. Besides, the poor contrast images can't be used for virtual endoscopy reconstruction (VE and VR) and analysis. Therefore, the development of negative CT contrast agent (with low CT density) is needed for clearer intestinal wall visualization, and 3D virtual endoscopy reconstruction and analysis.
Up to date, there is no such negative contrast agent available clinically that can satisfy all three requirements in CT imaging for the GI tract: good filling of the intestine, clear visualization of the intestinal wall and feasible for 3D virtual endoscopy. There were some reports proposing the use of plant cellulose, milk, emulsion, and paraffin to be used as the negative CT contrast agents. The CT densities of the plant cellulose, milk and fat emulsion are in the range of 10 to −80 Hounsfield Unit (HU), which is not low enough for clear visualization of the intestinal wall and the 3D virtual endoscopy reconstruction. The CT density of paraffin is around −100 HU. It is useful for the better depiction of the intestinal wall and 3D virtual endoscopy reconstruction, but severe diarrhea resulting from the administration of paraffin restricts its clinical application. Air filling is widely used in clinical practices to serve as a kind of negative contrast medium. It has the CT density of −1000 HU, which is optimal for 3D virtual endoscopy reconstruction and can detect extruding features in the lumen such as polyps or tumors with higher sensitivity and specificity, but the depiction of the intestinal wall in the transverse images is poor because of the air/water interface distortion. The thickness of the intestinal walls looks much thinner, so it is very hard to find any pathological changes on the intestinal wall.
Technological literature searches show that the only contrast agent that has been used in the clinic and actually improved the CT visualization of the intestinal wall was the Mucofalk suspension made of the swelling plantain seeds. (Helical CT of the small bowel with an alternative oral contrast material in patients with Crohn disease, Doerfler O C, Ruppert-Kohlmayr A J, Reittner P, et al, ABDOMINAL IMAGING, 2003, 28(3): 313), but the CT density difference between the intestinal wall and the lumen is still not large enough for 3D virtual endoscopy.